At least two South Korean refiners -- SK Energy Co. and S-Oil Corp. -- are competing to take over an Australian oil company, according to their respective stock market notices released over the last few weeks. SK Innovation Co., the owner of the largest local refiner SK Energy, said Monday in a regulatory filing that its refinery unit joined the race to acquire stakes in United Petroleum Pty., an Australian petrol and convenience store retailer, in a bid to explore markets overseas.